This invention is generally in the field of gas turbine power generation systems. More particularly, the present invention is directed to a system and method of cooling turbine airfoils with a stream of carbon dioxide from a carbon dioxide source.
Combustion turbines are often part of a power generation unit. The components of such power generation systems usually include the turbine, a compressor, and a generator. These components are mechanically linked, often employing multiple shafts to increase the unit's efficiency. The generator is generally a separate shaft driven machine. Depending on the size and output of the combustion turbine, a gearbox is sometimes used to couple the generator with the combustion turbine's shaft output.
Generally, combustion turbines operate in what is known as a Brayton Cycle. The Brayton cycle encompasses four main processes: compression, combustion, expansion, and heat rejection. Air is drawn into the compressor, where it is both heated and compressed. The air then exits the compressor and enters a combustor, where fuel is added to the air and the mixture is ignited, thus creating additional heat. The resultant high-temperature, high-pressure gases exit the combustor and enter a turbine, where the heated, pressurized gases pass through the vanes of the turbine, turning the turbine wheel and rotating the turbine shaft. As the generator is coupled to the same shaft, it converts the rotational energy of the turbine shaft into usable electrical energy.
Cooling design plays a very important role in turbine efficiency. Modern gas turbine systems are fired at temperatures that often exceed the melting temperatures of turbine materials. This is done because higher Brayton cycle thermodynamic efficiencies can be gained at higher firing temperatures. In order to operate at these elevated temperatures, cooling technologies must be employed to protect hot gas path components. The turbine blades and vanes are of particular concern because they are directly exposed to the hottest gases. Cooling design of the hot gas path components typically focuses on (1) internal convective cooling, (2) external surface film cooling, (3) material selection, (4) thermal-mechanical design, and (5) selection of coolant fluids. Research and development resources have been dedicated to each of these areas to a varying extent, with coolant selection historically receiving the least amount of consideration. When selecting a coolant fluid, it is important to select a coolant fluid that is capable of performing the needed cooling functionality while having the least negative effect on Brayton cycle efficiency. Compressor bypass air and steam are the most widely used coolant fluids.
Unrelated to cooling design, carbon dioxide sequestration techniques are increasingly being employed in power generating stations. Social and political pressures to reduce emissions of carbon dioxide have been the primary driving forces for the implementation of these processes. Power generating stations that utilize carbon dioxide sequestration incur a significant energy cost related to the carbon dioxide separation and storage. As such, it would be desirable to provide a cost-offsetting use of the sequestered carbon dioxide.